I tried to make my own boundary condition, and discovered, that the mixed boundary condition does not behave as expected. With the following set, I thought it would work like 'zeroGradient' or 'fixedGradient' with gradient=0, but it does not. The Poisson equation for p does not converge ...

type mixed;
gradient 0;
value 0;
valueFraction 0;

Any hints? Mixed boundary condition is a mix of Dirichlet and a normal gradient?